The series is inspired by the epic sagas about the raiding, trading, and exploring Norsemen of early medieval Scandinavia. It follows the exploits of the legendary Viking chieftain Ragnar Lodbrok and his crew and family.

The first season portrays Ragnar (Travis Fimmel) as a young Viking warrior who longs to discover civilizations across the seas. With his friend, the gifted craftsman Floki (Gustaf Skarsgård), he builds a new generation of faster longships and challenges the local ruler, Earl Haraldson (Gabriel Byrne), a man of little vision, to allow raids into the unexplored South West, to England. He is supported by his brother Rollo (Clive Standen), who secretly covets Ragnar's wife, the shieldmaiden Lagertha (Katheryn Winnick). Ragnar succeeds in carrying out the first Viking raids into the British kingdom of Northumbria, returning with rich loot and the monk Athelstan (George Blagden) as a slave. This triggers a series of increasingly violent confrontations with the autocratic Earl.